1. Field of the Invention
The invention relates to a method and apparatus for inspecting wind turbine blades and power generating equipment in rotating wind turbine generators.
2. Description of the Related Technology
Due to their large size and extensive surface area and complex shape, wind turbine blades are difficult to non-destructively inspect even in the factory. Visual inspection does not see defects below the surface. Active thermography inspection techniques are effective for near surface defects but can give false positives and false negatives due to variations in material thickness and surface emissivity. Angle beam ultrasonic techniques are very slow and may not work through thick carbon fiber spar caps. As a result, blades are installed on towers and put into service with a significant probability of latent manufacturing defects.
Furthermore, composite blades are subject to extreme temperature variations. Entrapped water in blades can undergo freeze/thaw cycles causing internal damage. Cyclic forces of gravity and varying forces from the wind acting on the blades as they rotate can cause fatigue damage or the propagation of latent defects over time while manufacturing process mistakes can lead to early blade failure. Defects can grow below the surface of blades to the point that by the time cracks and damage breach the surface and can be detected visually, the damage may not be repairable on tower.
Detecting progressive subsurface damage and propagating defects in wind turbine blades in situ is difficult. Inspectors using sky cranes or rope access are expensive, time consuming and put personnel in a very dangerous working environment. Access with portable instruments for nondestructive testing again requires rope access or sky platforms and cranes. Blade and tower crawlers with nondestructive testing sensors for in situ inspection have been developed and tested, with high cost implications, slow inspection rates and questionable effectiveness. While on tower, close access allows inspectors to visually detect such blade defects as trailing edge splits, cracks, lightning damage and blade erosion.
There accordingly exists a growing need for a fast, cost effective nondestructive inspection methods for wind turbine blades to detect these and other anomalies.